Most drug discovery programs begin with a screening campaign (e.g. biochemical, virtual or biophysical) for agonists, antagonists or inhibitors of a nominated target associated to a particular disease [1-4]. After hit identification, subsequent chemical optimization is fundamentally based upon “on-target” potency [1]. Generation of so-called lead compounds (=high-affinity ligands) is followed by medicinal chemistry refinement into derivatives of superior potency and/or selectivity, and desirable pharmacokinetic properties (=druglikeness) [1, 5]. Selected drug candidates are then validated in vivo and, upon verified safety and efficacy, progressed to human trials [5]. This well-defined process, which typically consumes over a decade of research work and tens of millions of pounds on its path to the clinic, is finding a particularly low success rate in the development of anticancer drugs [6]. This is because, on top of the enormous difficulties of translating a drug discovery program from target identification—through preclinical and clinical development—into regulatory approval and marketing, it has become apparent that conventional approaches are not appropriately tailored to pathologies generated by the concurrent or sequential action of multiple etiologic factors such as cancer [6-8]. High attrition during late-stage drug development has underlined that elucidating cancer heterogeneity across patients and adaptive drug resistance mechanisms are the major obstacles to the development of effective targeted anticancer therapies [9-11]. These challenges are stimulating out-of-the-box thinking in pharmacotherapy research (e.g. targeted polypharmacology [10], antibody-drug conjugates [12], innovative prodrug approaches [13-17], etc.) and the reexamination of the core principles of drug discovery in oncology [18-20]. The rise of modern phenotypic drug discovery [18, 19] together with the use of clinically-relevant cancer models to guide early drug development [20], are representative examples of the paradigm shift initiated in the field to trigger a positive inflection point.
Protein kinases are integral components of intracellular signal transduction cascades. They govern a wide range of basic cellular functions and coordinate cell-to-cell and extracellular matrix (ECM)-to-cell communications to influence cell and tissue physiology. Thereby, kinases are directly involved in progressive diseases including cancer and inflammation [21]. Advances in the understanding of cancer cell biology along with the approval of several kinase inhibitors for cancer treatment have demonstrated the validity of a number of kinases as anticancer targets [22], while, on the contrary, other protein kinases have been shown to play an essential role in tumor suppressor pathways (anti-targets) [23-26].
The vast majority of kinase inhibitors target the kinase ATP pocket and because all kinases (>500) necessarily possess this relatively well-conserved catalytic site, there is a great potential for cross-reactivity [10]. In fact, even if most clinically-approved kinase inhibitors have been developed from a single target hypothesis, they typically display a broad selectivity profile which, in some cases, have resulted in unanticipated clinical applications (e.g. sorafenib) [26]. An inhibitor's promiscuity may also be advantageous for anticancer therapy when off-target activities assist to address bioactivity issues related to pathway redundancies, molecular heterogeneity or resistance mechanisms [9, 10, 26]. On the contrary, if these activities result in the inhibition of anti-oncogenic pathways or lead to severe side effects, drug promiscuity becomes a major drawback. Paradoxically, there is strong evidence indicating that some kinases may behave as a target or an antitarget depending of the cancer context. By way of illustration, the expression of the activated fusion oncoprotein Bcr-Abl is a genetic abnormality associated with chronic myeloid leukemia (CML) and Abl inhibitors (imatinib, dasatinib) are clinically used in chronic phase CML treatment [27]. In addition, Abl family kinases are abnormally activated in various solid tumors, supporting their involvement in oncogenesis [27]. However, Abl (Abl1) and Arg (Abl2) have been found to negatively modulate breast cancer progression in vivo [28-30], indicating that Abl inhibition could be counterproductive for its treatment (=breast cancer anti-target). This example serves to delineate the complexity of cancer etiology and highlights the necessity of developing kinase inhibitors with tailor-made pharmacodynamic profiles for the effective targeting of each cancer subtype. Unfortunately, despite the vast amount of small molecule inhibitors and biomedical knowledge built over the years, the limited understanding of cancer biology prevents the appropriate targeting of orchestrated actions that generate, maintain and progress most neoplastic processes.
Acknowledging these limitations, many research groups are frontloading the search of robust empirical data to progress anticancer drug development programs away from classical black-and-white anticancer hypotheses.
The present invention seeks to provide tyrosine kinase inhibitors having potent antiproliferative properties. The invention is founded on three hypotheses: (i) the use of phenotypic screening in designated models of cancer can be used to generate target-agnostic structure-bioactivity relationships and guide ligand optimization tailored to particular cancer types/subtypes; (ii) targeting the kinase ATP pocket with compounds derived from promiscuous kinase inhibitors can enable “rationally-biased” serendipitous discoveries; and (iii) early improvement of druglikeness on promiscuous ligands can be concurrently used to explore pharmacodynamic diversity. By means of this pragmatic approach to kinase inhibitor discovery, target deconvolution of identified hits and leads was largely facilitated, thereby enabling the rapid identification of the molecular targets and antitargets involved in the observed phenotype.